Photoinduced disproportionation of Ru(bpy)(3)(2+) cation exchanged onto porous Vycor glass occurs within a fixed array of immobilized adsorbates by means of a mobile, photodetached electron. As the moles of complex adsorbed increases, the mean separation between the adsorbed ions decreases and approaches the electron migration distance. As a result, the quantum efficiency of disproportionation initially increases with loading, reaches a maximum, and then declines as adsorbate spacing approaches the distances over which the thermal back-reaction occurs. Surprisingly, expressing this Gaussian-like dependence of the quantum efficiency on initial RU(bpy)(3)(2+) loading as a difference between the probability of another redox partner within the electron migration distance, less the probability that the redox partners are within the thermal back-reaction distance, where the probabilities are expressed as exponentials, fails to agree with the experimental data. Introducing a fractal dependence by scaling the distance parameters also fails to achieve agreement. Results presented here show that agreement with the experimental data requires considering the mechanism of electron conduction. In the absence of an accessible conduction band, electron migration on this glass is thought to occur via the population of surface acceptor sites. Treating these sites as shallow energy wells that reduce the energy and migration distance of the photodetached electron leads to excellent agreement with the observed dependence.